1. Field of the Invention
The present invention relates generally to acoustic signal level detection and control. More specifically, systems and methods for detecting and variably and/or shiftably limiting the acoustic signal output level are disclosed.
2. Description of Related Art
Proper control of acoustic signal levels in communications and other audio output devices is desirable to ensure high quality audio output and hearing safety to the users. For example, a telephone headset provides a speaker contained within an earpiece positioned over the user's ear. To ensure acoustic safety and high acoustic quality, the sound level of the acoustic signal emitted by the speaker should fall within a specified sound intensity range. Below the specified intensity range, the inadequate sound level may not be sufficient for the user to hear or understand what is being uttered by the remote talker. On the other hand, above the specified intensity range, the excessive sound level may cause discomfort for the user and/or injury to the user's hearing. Thus excessively high sound levels are of particular concern in communication and other audio devices such as telephone handsets and headsets and other listening devices that position a speaker near the user's ear.
Excessively high sound levels may be caused by various events. For example, accidental disturbances within a communication connection, such as a amplifier malfunction, intense feedback, incorrect signal source, and/or a phone line shorted to a power line, may cause dramatic increases in the electrical signal level input to a transducer that converts electrical signals to acoustic signals. The transient time for the acoustic signal to reach excessively high levels may be very short such that a user often does not have sufficient time to move the listening device away from the ear in time to prevent exposure to the high sound levels. Although a handset user may be able to quickly move the handset speaker away from the ear as the user is typically already holding the handset in the hand, it may take a hands-free headset user longer to bring the hand to the headset in order to move the headset earpiece away from the ear. Furthermore, headsets are particularly suitable for users who are on the telephone for long periods of time, e.g., telemarketers, receptionists, and operators. Thus because of the extra time required to remove a headset away from the ear and the potentially longer periods of headset usage, headset users may be particularly vulnerable to exposure to excessively high sound levels caused by sudden or constant loud audible signals.
Headsets and other audio output devices often employ audio limiting devices on the receiver input terminals in order to limit the voltage and thus the maximum sound level from the headset receiver. Examples of audio limiting devices include a pair of diodes in shunt arranged in opposing polarity (as shown and described below with reference to FIG. 1) and an acoustic limiting component utilizing transistors (as shown and described below with reference to FIG. 2).
In particular, FIG. 1 is a circuit diagram illustrating an exemplary signal circuit 20 employing a conventional varistor circuit 22 for reducing the maximum signal level to the receiver and thus the sound exposure to the user. The varistor circuit 22 is essentially a surge protector that creates a shunt path using diodes to attenuate signal peaks. As shown, a voltage source Vs 24 with a corresponding source impedance Rs 26 provides an electrical signal to an attached load RL 28 via the varistor circuit 22. The voltage source 24 may be, for example, a telephone or telephone adapter and the attached load 28 may be a receiver such as a telephone headset or handset speaker.
The varistor circuit 22 includes a first diode D1 30 in parallel to a second diode D2 32, each having a corresponding turn-on voltage, e.g., 0.5 to 0.7 volts, to activate the corresponding diode. Each diode 30, 32 shunts the respective positive or negative portion of an electrical signal through its corresponding path after the respective diode turn-on voltage is reached by the electrical signal received from the voltage source 24. In particular, the first diode 30 is activated to shunt the negative portion of the electrical signal current when the magnitude of the negative portion of the electrical signal current exceeds the first diode's turn-on voltage. In other words, when the magnitude of the negative portion of the electrical signal current is below the turn-on voltage, the first diode 30 remains cut off. However, when the magnitude of the negative portion of the electrical signal current is above the turn-on voltage, the first diode 30 turns on to conduct current from the voltage source 24, thereby creating a shunt path. The specific amount of current conducted through the shunt path depends on the impedance of the load 28. As a result of the activated negative shunt path, the level of the negative portion of the electrical signal current received by the load 28 is reduced such that the output voltage Vout 34, i.e., the relative voltage across the load 28, is similarly reduced.
The second diode 32 operates in a similar manner as the first diode 30 except that it creates a shunt path for the positive portion of the electrical signal received form the voltage source 24. As a result of the positive and negative diode shunt paths, the magnitude of each positive and negative portion of the electrical signal and the output voltage Vout 34 are attenuated when the diodes 30, 32 are turned on.
FIG. 2 is a circuit diagram illustrating another exemplary signal circuit 40 employing a discrete transistor circuit 42 in parallel with a varistor circuit 22 for reducing electrical signal peaks. As shown, the discrete transistor circuit 42 is in parallel with the first and second diodes 30, 32 of the varistor circuit 22. The discrete transistor circuit 42 includes a first transistor Q1 44, a second transistor Q2 46, a first resistor R1 48 and a second resistor R2 49. The base and emitter of the first transistor 44 are connected across the voltage source 24. The second transistor 46 is coupled in parallel to the voltage source 24 in a similar manner. The discrete transistor circuit 42 is placed in front of the two diodes 30, 32 and is activated by an electrical signal voltage level from the voltage source 24 being above the turn-on voltage of the first and second transistors 44, 46.
The first transistor 44 attenuates the positive portion of an electrical signal after the voltage level between the emitter and base of the first transistor 44 exceeds the turn-on voltage of the first transistor 44, e.g., approximately 0.5-0.7 volts. Once the first transistor 44 is turned on, an attenuation network is created that includes the first resistor 48 and the resistance Rce1 between the collector and emitter of the first transistor 44. The attenuation network decreases the positive voltage at the load 28 by allowing current to flow through the first transistor 44. As the voltage level of the electrical signal from the voltage source 24 increases further above the turn-on voltage, the first transistor 44 reaches saturation mode and the resistance Rce1 decreases. As a result, because Rce1 forms a divider network with the first resistor 48, the current flowing through the first transistor 44 increases, thereby further limiting the relative voltage across the load 28. Thus increases in the voltage level of the voltage source 24 above the turn-on voltage of the first transistor 44 results in deepening of the saturation level within the first transistor 44 and a decrease in the resistance Rce1 such that the voltage across the load 28 decreases.
The second transistor 46 operates in a similar manner as the first transistor 44 but on the negative portion of the electrical signal from the voltage source 24. Specifically, after the second transistor 46 is turned on, an attenuation network is created including the second resistor 49 and the resistance Rce2 between the collector and emitter of the second transistor 46. The second transistor 46 thus operates to attenuate the negative portion of the electrical signal to limit the relative voltage across the load 28 as the negative voltage level of the voltage source 24 increases.
However, receivers often differ in receiver sensitivity and impedance. Accordingly, it would be desirable to provide an acoustic limiting device that varies the attenuation level of an audio listening device depending on the level of the received voltage. In particular, it would be desirable to provide an acoustic limiting device that provides less signal attenuation in a linear range and greater signal attenuation in the non-linear range.
In addition, headsets are becoming more commonly used for multiple applications, e.g., both speech such as in traditional telephony and music. Peak to average acoustic level ratios for music are greater than those for speech such that traditional diode limiting causes undesirable audible distortion for louder music listening applications. Limiting diodes may be selected to suit each application, e.g., voice and music. For example, diodes may be provided for the voice applications and removed and/or switched off for music applications. However, such removal and/or switching off the limiting diodes would not provide audio limiting protection for music applications. Alternatively, a switch may be provided to select diodes corresponding to the particular application thus requiring redundant diodes.
It would thus also be desirable to vary the audio limiting level depending on the application for which the headset is used, e.g., a higher audio limiting level for music applications and a lower audio limiting level for voice applications.